It is known in the art that an optical Kerr shutter may be used to provide an optically sampled optical "probe" output signal of a given wavelength (.lambda.s) from a continuous wave (CW) optical "probe" input signal with the same wavelength (.lambda.s) and an optical "pump" input signal of a different wavelength (.lambda.p) pulsed at the desired sample frequency.
The Kerr shutter, as is known, operates on the principle of the optical Kerr effect. The optical Kerr effect is a property of some materials that causes a change in the index of refraction of the material in the presence of a light beam (or pump). The Kerr shutter uses this property to induce birefringence, i.e., non-uniform refractive indices along different axes of an optical waveguide medium, between two axes of propagation by injecting an optical pump. The pump alters the index of refraction (n) in the direction it is polarized while it is present (due to the Kerr effect). In an all-optical Kerr shutter, the pump signal and the probe signal (referred to hereinafter as "pump" and "probe") are combined by known optical means, and launched into one end of a polarization preserving (or maintaining) optical fiber (i.e., prevents the components along the fiber axes from scrambling). Typically, the pump has a polarization direction along one axis and the probe has a polarization direction 45.degree. from the pump. More specifically, the pump typically has a polarization along the x axis and the probe has a polarization of 45.degree. between the positive x axis and the positive y axis which decomposes into components of equal magnitude along the x and y axes, and the pump is pulsed at a given frequency. When the pump pulse is not present, the components of the probe along the x and y axes propagate through the waveguide medium (polarization preserving fiber) having the same index of refraction (n) along both axes. Thus, the signals exit the output end of the fiber exhibiting no phase shift between them (as they were when they entered the fiber).
An analyzer (or polarizer) is typically placed at the output end of the fiber to monitor the waves exiting the fiber and provides an optical output signal indicative of the component of the polarization direction of the probe signal (i.e., the resultant time varying vector sum magnitude of the probe components along x and y axes of propagation) that lies along the analyzer output (or "fast" or transmission) axis. For example, if the analyzer transmission axis is configured to pass a signal with a polarization direction of -45.degree. from the positive x axis, i.e., 90.degree. from the polarization of the probe input signal, the output of the analyzer will be zero (when the pump pulse is not present) because no component of the resultant magnitude vector from the x and y component waves lies along the analyzer transmission axis.
When the pump pulse is present, the index of refraction in the x direction (n.sub.x) will not equal the index of refraction in the y direction (n.sub.y) due to the aforementioned optical Kerr effect. The change in n.sub.x will cause a change in the propagation time for the x-axis wave component of the probe signal thereby causing the component to be shifted out of phase from the y-axis wave component of the probe signal (whose index of refraction was unchanged by the presence of the pump pulse). This phase shift produces a new vector sum of the probe wave components at the output end of the fiber, i.e., a new polarization of the probe signal, that is shifted from the probe polarization that existed when the pump was not present, the polarization change being related to the amount of phase shift caused by the induced change in refractive index. Therefore, when the pump pulse is present, the Kerr effect induces a change in polarization of the exiting probe signal.
Ideally, the polarization change of the probe due to the pump is 90.degree. thereby becoming exactly parallel with the analyzer transmission axis and providing full probe signal strength at the analyzer output. If the polarization change is less than 90.degree., then the maximum signal strength will be less than the input probe signal strength because the component of the polarization along the analyzer transmission axis is a smaller magnitude. Thus the pump induced change in polarization of the probe signal allows a predefined portion of the probe signal intensity to be transmitted through the analyzer.
If a 90.degree. polarization change is achieved when the pulse is present, the analyzer output will represent a sample of the probe signal (i.e., the value of the probe signal input when the pump pulse occurs). However, when the pump pulse goes to zero, the polarization change is zero and the analyzer output goes to zero. Therefore, the Kerr shutter provides a signal which represents the input signal optically sampled at the rate (frequency) at which the pump pulses occur.
The optical Kerr shutter behaves similarly to an electronic sampler of an analog signal where the signal to be sampled is the probe and the sampler is driven by the pump. If the Kerr shutter output reproduces the full power of the input signal at each sample, it is known as 100% Kerr modulation (or 100% probe transmission), i.e., 100% of the probe signal power is reproduced at the output. The Kerr shutter may also be viewed as a wavelength converter because it converts a pulsed optical input signal (pump) at one wavelength (.lambda.p) to a pulsed (sampled) output signal (probe) at a different wavelength (.lambda.s). The optical Kerr shutter described herein is similar in principle to that described in the article: Ken-ichi Kiayama et. al., "Optical Sampling Using an All-fiber Kerr Shutter", Appl. Phys. Lett., American Institute of Physics, Vol. 46, No. 7, (April 1985).
In optical fiber communications and optical switching it is desirable to achieve a controlled pulsed signal at a desired wavelength, e.g., telephone communications, cable television, and optical computing.
In the past, to provide this function, devices have used two optical signals; the pump and the probe (as previously discussed), thus requiring two signals to produce one. Furthermore, the pump power was only used to drive the optical Kerr shutter and discarded at the output end of the fiber, thereby wasting optical pump power.